1. Field of the Invention
The present invention generally relates to the management of battery charging and discharging, and more particularly to a lithium battery protection circuit that provides proper charging and discharging so that optimum battery life and performance are obtained.
2. Description of Related Art
The number of electronic devices that are available in portable form has continued to increase over the years. These portable devices include cellular telephones, radios, pagers, voice recorders and the like. In order to provide portability, these electronic devices are generally configured to operate using a rechargeable battery. While many different battery technologies have been utilized, lithium ion batteries have characteristics that make them a preferred rechargeable cell.
The benefits of a lithium ion battery include high energy density, low weight and small overall size. However, while these characteristics can be seen to be advantageous in a portable setting, the unique operating requirements of a lithium ion cell must be addressed to effectively exploit this technology. Specifically, in order to achieve optimum battery life and performance, the potential of a lithium ion battery must be maintained within an operating range as defined by a lower threshold voltage and an upper threshold voltage. As this operating range is crucial in extending the battery life as well as performance, dual protection has been employed to ensure the battery potential remains within the desired range. Generally, this is accomplished by using both a protection circuit included with the battery charger, and a battery manager that is formed as an integral part of the battery. An illustration of a prior art battery manager 20 is shown in FIG. 1.
Referring to FIG. 1, it can be seen that the battery manager 20 has a controller 24 that passively monitors the potential of the battery 22. Based upon this measured value, a voltage is received by a first gate 26 of a first n-channel enhancement Field Effect Transistor (FET) 28 and a second gate 30 of a second n-channel enhancement FET 32 disposed in series with the first FET 28. The voltage presented to the FET gates 26,30 configures the FETs 28,30 such that flow from the charger 34 to the battery 22 and the current flow from the battery 22 to the load 36 (e.g., cellular telephone) is regulated to maintain the voltage of the battery 22 between an upper threshold voltage and a lower threshold voltage.
In operation, a voltage is applied to the first gate 26 and second gate 30 when the voltage of the battery 22 is between the upper threshold and lower threshold. Therefore, the first FET 28 and second FET 32 are active and the battery 22 is connected to the cellular phone 36 and charger 34. In this manner, the battery 22 can draw upon the charger 34 or the cellular phone 36 can utilize the battery 22 if the charger 34 is unavailable. However, if the battery 22 is approaching the upper or lower threshold, application of a voltage to the first gate 26 or second gate 30 is discontinued so that current flow is restricted and battery charging or discharging is terminated.
The two FETs 28,32 restrict current flow in conjunction with a first diode 38 reversed in polarity with respect to a second diode 40. If the voltage of the battery 22 approaches (or achieves) the upper threshold, voltage application to the second gate 30 is discontinued and any current is forced to flow through the second diode 40. This terminates current flow from the charger 34 to the battery 22 and allows current to flow only from the battery 22 to the cellular phone 36 as necessary to operate the device (i.e, charging is discontinued and the battery 22 is available to the cellular phone 36).
Conversely, if the battery 22 approaches (or hits) the lower threshold, the voltage is not applied to the first gate 26 and any current is forced to flow through the first diode 38. Consequently, current will cease to flow from the battery 22 to the cellular phone 36, but the battery 22 will be able to draw upon the charger 34 (i.e., battery charging is available while the cellular phone 36 is unable to use the battery 22 as a power source). While this configuration adequately controls the discharging and charging of the battery 22 so that the battery potential remains within the desired range, significant disadvantages exist.
Prior to the present invention, battery managers were manufactured with discrete devices. This was primarily due to FET isolation requirements as undesirable leakage was possible when multiple FETs were contained in the same substrate. However, the use of discrete devices significantly increases production costs and unit complexity. Furthermore, as a small path resistance between the charger and the load is desirable to avoid excess power dissipation in the current transmission from the battery to the load or from the charger to the battery, large area transistors were required in a series combination so that a minimum path resistance (e.g., 50 milli-ohms) could be achieved. However, FETs encompassing large areas tend to increase manufacturing costs and limit efforts to achieve minimum package sizes.
An improved battery manager is thus needed which effectively maintains a battery voltage within a specified voltage range in a cost effective manner yet addresses the concerns of transistor leakage, power dissipation, and component size.